" WIND-SCORPIONSr 331 



only squeezed and depressed the prostomium or upper Up 

 between them but entirely altered the shape of the first 

 segment to which they belonged. The sides of the 

 ■segment on which they were originally inserted with the 

 surrounding regions to which their muscles were attached 

 must have shifted from their original positions and become 

 the bases of the limbs in their new positions. The lateral 

 regions of the first segment were thus gradually folded 

 upwards over its dorsal surface. In time as the limbs 

 grew in size and strength and required larger and deeper 

 bases, these folds spread backwards over the dorsal surfaces 

 •of the second and part of the third segments, the two 

 folds eventually meeting in the middle line, obliterating the 

 old dorsal surface with the exception of one small island 

 which carried the eyes. In all existing Arachnids the 

 lateral folds of the first segment, carrying the first pair of 

 limbs, have met along the middle line, the junction being 

 .still marked by a suture through which in many forms the 

 ocular tubercle protrudes ; but the fact that a fossil scorpion 

 from the Silurian shows an irregular shaped island of the 

 dorsal surface not yet covered over by these folds makes it 

 doubtful whether they ever actually met in the ancestral 

 form. The meeting may have been a later specialisation 

 of the derived forms. 



3. This translocation of the first pair of limbs to a position 

 above the mouth made room for the second pair to come 

 forward also. This they did, and took up positions at the 

 sides of and slightly below the mouth, their coxal joints 

 pointing forwards. In some cases {(■'.£'., Thelyphonus) they 

 have fused longitudinally below the mouth just as the 

 basal regions of the first pair fused together above the 

 mouth. 



4. The anteriorly placed mouth was thus in time sur- 

 rounded by limbs, two above and two below, and its shape 

 became specialised accordingly. The prostomium squeezed 

 between the jaws fused along its lateral edges with the 

 ■edges of the underlip which had been forced to protrude 

 anteriorly by the forward movement of the limbs of the 

 second segment. In this way a sort of rigid beak was 



